Nasa detected a burst of positrons with the Fermi detector back in 2010 that originated from earth. They think it comes from "dark lightning," a chain reaction of high energy electrons emitted from a storm hitting air molecules and producing gamma rays. Gamma rays in turn interact with more air molecules producing electron - positron pairs, and the cycle continues. Since the process mostly takes place outside of the visible spectrum, you get an invisible burst of antimatter detectable from orbit.